This application claims priority to European Patent Application EP 01306260.9 filed Jul. 20, 2001, which document is herein incorporated by reference.
The present invention relates to lithographic projection apparatus and methods.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to any structure or field that may be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of a substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, such a pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of very small (possibly microscopic) mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. For example, the mirrors may be matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and No. 5,523,193, which are incorporated herein by reference, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (e.g. a wafer of silicon or other semiconductor material) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system (e.g. one at a time). Among current apparatus that employ patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. A projection beam in a scanning type of apparatus may have the form of a slit with a slit width in the scanning direction. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, which is incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
The term xe2x80x9cprojection systemxe2x80x9d should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d. The radiation system may also include components operating according to any of these design types for directing, shaping, reducing, enlarging, patterning, and/or otherwise controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT Application No. WO 98/40791, which documents are incorporated herein by reference.
In many lithographic projection apparatus, the substrate (for example, a wafer) must be delivered to the substrate table within a predetermined range from a predetermined position and with a known translational and rotational offset from the predetermined position. This delivery is typically performed by a pre-alignment unit (or xe2x80x9cprealignerxe2x80x9d) which is part of a wafer handling system. The prealigner determines the orientation of the wafer with respect to the prealigner, positions the wafer such that it will arrive at the wafer table within specification, and determines what the remaining offset from the predetermined position will be.
It has previously been known to pre-align wafers by mechanically pressing one edge or corner of the wafer against a known surface or pair of surfaces. However, this method is relatively slow and may introduce contamination to the wafer. Additionally, the accuracy achievable by such a method may be relatively low. Factors such as wear and wafer expansion may further reduce the accuracy, and the method can result in chipping of the edge of the wafer, which may also further reduce the accuracy. This method cannot detect notches (e.g. which may be used to mark orientation of the wafer) and is also typically limited in the range of wafer sizes that can be handled.
A prealignment system according to one embodiment of the invention is configured to determine a position and orientation of a substrate (e.g. a polygonal substrate). The system includes means for rotating a substrate about an axis of rotation that is substantially perpendicular to the plane of the substrate. The system also includes a non-contact edge sensor configured to indicate, at each of a plurality of angles of rotation of the substrate, a corresponding distance of an edge of the substrate along a line intersecting the axis of rotation. Means are also provided for determining, based on the plurality of distances, best-fit lines for at least two edges of the substrate, and for determining a position and orientation of the substrate based on the best-fit lines.
A lithographic projection apparatus according to another embodiment of the invention includes a prealignment system as described above.
A device manufacturing method according to an embodiment of the invention includes using a radiation system to provide a projection beam of radiation, using patterning structure to endow the projection beam with a pattern in its cross-section, and projecting the patterned beam of radiation onto a target portion of a layer of radiation-sensitive material that at least partially covers a substrate on an object table. The method includes, prior to projecting the patterned beam of radiation onto the target portion, determining a position and orientation of the substrate. In this embodiment, determining a position and orientation of the substrate includes rotating the substrate about an axis of rotation substantially perpendicular to the plane of the substrate; indicating, at each of a plurality of angles of rotation of the substrate, a corresponding distance of an edge of the substrate along a line intersecting the axis of rotation; determining, based on the plurality of distances, best-fit lines for at least two major edges of the substrate; and determining the position and orientation of the substrate based on the determined best-fit lines.
Embodiments of the invention also include computer programs for calculating a position and orientation of a substrate, and computer programs for operating a lithographic projection apparatus. For example, a data storage medium (e.g. a magnetic or optical storage medium such as a disk; a volatile and/or nonvolatile memory unit such as RAM, DRAM, SDRAM, ROM, or flash; etc.) according to one embodiment of the invention has machine-readable code, the machine-readable code including instructions executable by an array of logic elements, said instructions defining a method of calculating a position and orientation of a substrate. In this embodiment, calculating a position and orientation of the substrate includes rotating the substrate about an axis of rotation substantially perpendicular to the plane of the substrate; indicating, at each of a plurality of angles of rotation of the substrate, a corresponding distance of an edge of the substrate along a line intersecting the axis of rotation; determining, based on the plurality of distances, best-fit lines for at least two major edges of the substrate; and determining the position and orientation of the substrate based on the determined best-fit lines.
Although specific reference may be made in this text to the use of an apparatus according to an embodiment of the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus may have many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5-20 nm, especially around 13 nm), as well as particle beams, such as ion beams or electron beams.